Ambrosia L.
Annual or perennial herbs, rarely shrubs. Lvs opposite below, often alternate above, simple, lobed to 2-3-pinnatisect. Capitula unisexual, inconspicuous; all florets tubular. ♂ capitula sessile or pedunculate, in ebracteate spikes or racemes; involucre herbaceous, 5-12-lobed; receptacle flat; scales present. ♀ capitula each with a single floret, solitary or clustered, in axils of uppermost lvs; involucre enclosing achene, beaked at fruiting and often bearing 1-2 rows of spines or tubercules. Achenes all similar, ovoid, not winged; pappus 0.
Key
1
Annual; lf segments (1)-2-7 mm wide; fruiting involucre glabrous or sparsely hairy
Rhizomatous perennial; lf segments 0.5-1.5-(2) mm wide; fruiting involucre moderately to densely hairy
42 spp., N. and S. America. Naturalised spp. 2.
